Antistatic property relates, in general, to surface resistivity and charge amount. Reduced surface resistivity and small charge amount are desired and it is also preferred that these properties do not deteriorate with the passage of time.
A photographic light-sensitive material generally comprises a support such as film of poly-.alpha.-olefin (e.g., polyethylene, polystyrene, etc.), a cellulose ester (e.g., cellulose triacetate, etc.), a polyester (e.g., polyethylene terephthalate, etc.), paper, synthetic paper or a paper sheet coated on both sides with such high molecular weight materials or the like, coated on one side or both sides with a light-sensitive photographic emulsion layer (or layers) with an interposed subbing layer provided to strongly adhere the support to the photographic emulsion layers and, if desired or necessary, various layers constituting photographic light-sensitive materials such as interlayers, protective layers, a backing layer, an antihalation layer or the like in various combinations.
Examples of a photographic light-sensitive material having coated on both sides of a support a photographic emulsion include, for example, an X-ray film for direct use. Most other photographic light-sensitive materials comprise a support coated only on the side with a photographic emulsion.
Therefore, the latter type of photographic materials have a photographic emulsion-free surface, i.e., the surface of the support is usually called the back side. Since photographic light-sensitive materials comprise a support and photographic layers having an electrical insulating property, electrostatic charges tend to be generated and accumulated during the production of photographic light-sensitive materials and during use thereof due to contact friction with or delamination from the surface of the same or different substances. This accumulated electrostatic charge can cause many difficulties. For example, in a photographic film before development processing, the light-sensitive emulsion layer is sensitive to the discharge of the accumulated electrostatic charge and dot-like spots or tree-like or feathery linear spots are formed after developing the photographic film. These spots are generally called static marks. Such marks seriously deteriorate or, in some cases, completely spoil the commercial value of photographic films. For example, it can easily be seen that static marks appearing in an X-ray film for medical or industrial use would lead to extremely dangerous mis-recognition. This phenomenon is confirmed only after development, and is therefore an extremely difficult problem. In addition, the accumulated electrostatic charge causes adhesion of dust on the film surface and secondary difficulties such as non-uniform coating or the like.
As is described above, such electrostatic charge often accumulates during production of photographic light-sensitive materials and upon use. For example, in production, electrostatic charges are generated through contact friction between the photographic film and rollers or separation between the support surface and the emulsion-coated surface in the steps of winding or unwinding photographic films. Also, with finished products, electrostatic charges are generated by delamination of a base surface from an emulsion-coated surface when winding of the photographic film occurs under a humidity high enough to cause film adhesion, or by the contact and delamination of X-ray film from mechanical parts in an automatic processing machine or from a fluorescent sensitizing paper. In addition, charges may also be generated by contact with a wrapping material. The problem with static marks on photographic light-sensitive materials formed by the accumulation of such electrostatic charge increases greatly by increasing the sensitivity of the photographic light-sensitive materials and increasing the processing velocity.
These frictional charges or delamination charges are considered to be caused by a mutual ionic action between the molecules of substances contacted. However, at present, it is difficult to predict from structural and chemical viewpoints what substance will be negatively charged and what substances positively charged.
One solution, however, is to reduce the charge voltage or to increase the electric conductance on the surface of a substance to thereby release electrostatic charge in an extremely short time before localized discharge, due to the accumulation of electric charge occurs to prevent such charging. Therefore, processes have been suggested for improving the electro-conductivity of the support or various coated surface layers of photographic light-sensitive materials, and various hygroscopic substances, water-soluble inorganic salts, certain surface active agents, polymers or the like have been utilized. For example, polymers as described in, e.g., U.S. Pat. Nos. 2,882,157, 2,972,535, 3,062,785, 3,262,807, 3,514,291, 3,615,531, etc., surface active agents as described in, e.g., British Pat. No. 861,134, U.S. Pat. Nos. 2,982,651, 3,428,456, 3,457,076, 3,454,625, 3,552,972, 3,655,387, etc., zinc oxide, semiconductors, colloid silica, etc., as described in, e.g., U.S. Pat. Nos. 3,062,700, 3,245,833, 3,525,621, etc. are known for this purpose.
Known processes for directly imparting an antistatic property to photographic film supports include processes for directly compounding such substances in a support of a high molecular weight material and processes for coating the same on the surface of the support. In the latter case, an antistatic agent is coated as a backing layer by using it alone or in combination with gelatin, polyvinyl alcohol, cellulose acetate or a like high molecular weight substance.
Also, a process for preventing charging of photographic light-sensitive materials, is known comprising incorporating an antistatic agent in a photographic emulsion layer or a surface protecting layer thereof, or coating a solution of an antistatic agent on the surface of these layers. However, many of these substances are quite specific in their action depending upon the kind of film support or the composition of the photographic materials. Thus, a particular agent providing good results for certain film supports, photographic emulsions or other photographic elements is useless for antistatic purpose with respect to other different film supports and photographic elements, and, in some cases, exerts a detrimental influence on photographic properties.
In general, with high speed emulsions few antistatic agents provide satisfactory antistatic effects under condition of low humidity (about 30% RH), or, in many cases, a reduction in antistatic effects and an increase of adhesion difficulties are involved at conditions of high temperature and high humidity. In particular, with light-sensitive materials wherein a photographic emulsion is coated on both sides of a support (like X-ray light-sensitive materials for direct use), it has been difficult to establish techniques using effectively antistatic agents.
Further, it has been particularly difficult to establish stable antistatic techniques since in many cases antistatic properties which have been achieved tend to decrease with the passage of time or by friction.
Also, in searching for antistatic agents for photographic light-sensitive materials, it is necessary to take into consideration sensitivity, fog, granularity, sharpness and like photographic properties, as well as to consider maintaining a suitable friction constant and of not deteriorating camera behavior or adhesion resistance. Since it is very difficult to apply antistatic agents to photographic light-sensitive materials and the selection of antistatic agent is restrictive in many cases as described above, various applications of antistatic agents to photographic materials have been contemplated.